Rankine cycle apparatus and method for controlling rankine cycle apparatus

ABSTRACT

A Rankine cycle apparatus includes: a sensor; a pump; an evaporator; an expander; a condenser; and a fluid circuit through which a working fluid flows. The fluid circuit includes a circulation circuit in which the pump, the evaporator, the expander, and the condenser are provided in this order. The sensor is configured to detect one of (I) a pressure of the working fluid, (II) a temperature of the working fluid, and (III) a temperature of a coding medium to be heat-exchanged with the working fluid in the condenser. First control is started if a detected value of the sensor is less than a first threshold value, the first control is control to cause the pump to circulate the working fluid through the evaporator and/or a heater.

BACKGROUND 1. Technical Field

The present disclosure relates to a Rankine cycle apparatus and a method for controlling a Rankine cycle apparatus.

2. Description of the Related Art

Various types of Rankine cycle apparatuses have heretofore been studied. Japanese Patent No. 6179736 describes an example of a Rankine cycle apparatus.

FIG. 8 illustrates a Rankine cycle apparatus 100 according to Japanese Patent No, 6179736. In the Rankine cycle apparatus 100, a pump 101, an evaporator 102, an expander 103, and a condenser 104 are connected in an annular shape. The Rankine cycle apparatus 100 is provided with a bypass flow passage 110. The bypass flow passage 110 bypasses the expander 103. The bypass flow passage 110 includes a valve 105. The valve 105 regulates the flow rate of a working fluid in the bypass flow passage 110.

SUMMARY

One non-limiting and exemplary embodiment provides a technique suitable for ensuring the reliability of a Rankine cycle apparatus.

In one general aspect, the techniques disclosed here feature a Rankine cycle apparatus includes; a sensor; a pump; an evaporator; an expander; a condenser; and a fluid circuit through which a working fluid flows. The fluid circuit includes a circulation circuit in which the pump, the evaporator, the expander, and the condenser are provided in this order. The sensor is configured to detect one of (I) a pressure of the working fluid, (II) a temperature of the working fluid, and (III) a temperature of a cooling medium to be heat-exchanged with the working fluid in the condenser. First control is started if a detected value of the sensor is less than a first threshold value, the first control is control to cause the pump to circulate the working fluid through the evaporator and/or a heater.

The technology according to the present disclosure is suitable for ensuring the reliability of a Rankine cycle apparatus.

It should be noted that general or specific embodiments may be implemented as a system, a method, an integrated circuit, a computer program, a storage medium, or any selective combination thereof.

Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration diagram of a Rankine cycle apparatus according to a first embodiment;

FIG. 2 is a state diagram of a working fluid according to one example;

FIG. 3 is a flowchart illustrating control according to the first embodiment;

FIG. 4 is a configuration diagram of a Rankine cycle apparatus according to a second embodiment;

FIG. 5 is a flowchart illustrating control according to the second embodiment;

FIG. 6 is a configuration diagram of a Rankine cycle apparatus according to a third embodiment;

FIG. 7 is a flowchart illustrating control according to a fourth embodiment; and

FIG. 8 is a configuration diagram of a Rankine cycle apparatus according to an existing technology.

DETAILED DESCRIPTION Underlying Knowledge Forming Basis of the Present Disclosure

If the pressure of a working fluid is negative, air, moisture, or the like in the atmosphere is possibly mixed in a flow passage through which the working fluid flows. Preventing such mixing contributes to ensuring the reliability of the Rankine cycle apparatus.

Outline of One Aspect According to Present Disclosure

According to a first aspect of the present disclosure, a Rankine cycle apparatus includes: a sensor; a pump; an evaporator; an expander; a condenser; and a fluid circuit through which a working fluid flows. The fluid circuit includes a circulation circuit in which the pump, the evaporator, the expander, and the condenser are provided in this order. The sensor is configured to detect one of (I) a pressure of the working fluid, (II) a temperature of the working fluid, and (III) a temperature of a cooling medium to be heat-exchanged with the working fluid in the condenser. First control is started if a detected value of the sensor is less than a first threshold value, the first control is control to cause the pump to circulate the working fluid through the evaporator and/or a heater.

The technique according to the first aspect is suitable for preventing the pressure of the working fluid from becoming negative. This technique contributes to ensuring the reliability of the Rankine cycle apparatus.

According to a second aspect of the present disclosure, for example, in the Rankine cycle apparatus according to the first aspect, (i) the sensor may detect the pressure of the working fluid, and the first threshold value may be a pressure higher than or equal to the atmospheric pressure. Alternatively, (ii) the sensor may detect the temperature of the working fluid, and the first threshold value may be a temperature higher than or equal to the boiling point of the working fluid at atmospheric pressure. Still alternatively, (iii) the sensor may detect the temperature of a cooling medium to be heat-exchanged with the working fluid in the condenser, and the first threshold value may be a temperature higher than or equal to the boiling point of the working fluid at atmospheric pressure.

Each of (i), (ii), and (iii) of the second aspect is suitable for preventing the pressure of the working fluid from becoming negative.

According to a third aspect of the present disclosure, for example, in the Rankine cycle apparatus according to the first aspect or the second aspect, the sensor may detect the pressure of the working fluid in a portion of the circulation circuit that is downstream of the expander and upstream of the pump.

The third aspect is suitable for preventing the pressure of the working fluid from becoming negative.

According to the fourth aspect of the present disclosure, for example, in the Rankine cycle apparatus according to any one of the first to third aspects, the fluid circuit may include a bypass circuit configured to connect a portion of the circulation circuit that is downstream of the evaporator and upstream of the expander to a portion of the circulation circuit that is downstream of the expander and upstream of the condenser, and the working fluid may be circulated through the bypass circuit in the first control.

In the first control according to the fourth aspect, the working fluid can circulate by bypassing the expander by the bypass circuit. This configuration enables the working fluid to circulate smoothly in the first control.

According to a fifth aspect of the present disclosure, for example, in the Rankine cycle apparatus according to the fourth aspect, a valve may be provided in the bypass circuit, and an opening degree of the valve may be set to 50% or more and 100% or less in the first control.

According to the fifth aspect, in the first control, the opening degree of the valve of the bypass circuit is set to 50% or more and 100% or less. If the opening degree is set in this manner, the working fluid can be easily circulated smoothly in the first control.

According to a sixth aspect of the present disclosure, for example, in the Rankine cycle apparatus according to any one of the first to fifth aspects, the heater may be provided in the fluid circuit. The working fluid may be circulated through the evaporator by the pump in the first control. The heater may start generating heat if the detected value is less than a second threshold value and an elapsed time from the start of the first control is greater than or equal to a threshold time.

According to the sixth aspect, even if the risk of the pressure of the working fluid becoming negative cannot be sufficiently avoided by the first control, the risk can be avoided by using the heater.

According to a seventh aspect of the present disclosure, for example, the Rankine cycle apparatus according to any one of the first to sixth aspects may stop driving the pump if the detected value is greater than or equal to a second threshold value.

According to the seventh aspect, unnecessary power consumption of the pump can be avoided.

According to an eighth aspect of the present disclosure, for example, in the Rankine cycle apparatus according to any one of the first to seventh aspects, the boiling point of the working fluid at atmospheric pressure may be 0° C., or higher and 50° C. or lower.

If the boiling point of the working fluid is high to the extent specified in the eighth aspect, the pressure of the working fluid tends to become negative. Therefore, in this case, the technique of preventing the pressure of the working fluid from becoming negative is likely to exert its effect.

According to a ninth aspect of the present disclosure, for example, the Rankine cycle apparatus according to any one of the first to eighth aspects may further include an electric generator configured to generate electric power by the rotation torque of the expander.

According to the ninth aspect, electric power can be generated by the expander and the generator.

According to a tenth aspect of the present disclosure, a method is provided for controlling a Rankine cycle apparatus configured to circulate a working fluid through a pump, an evaporator, an expander, and a condenser in this order. The method includes detecting, by using a sensor, one of (I) a pressure of the working fluid, (II) a temperature of the working fluid, and (III) a temperature of a cooling medium to be heat-exchanged with the working fluid in the condenser and starting first circulation to cause the pump to circulate the working fluid in a heated state if a detected value of the sensor is less than a first threshold value.

The technique according to the tenth aspect is suitable for preventing the pressure of the working fluid from becoming negative. This contributes to ensuring the reliability of the Rankine cycle apparatus.

According to an eleventh aspect of the present disclosure, for example, in the control method according to the tenth aspect, (i) the sensor may detect the pressure of the working fluid, and the first threshold value may be a pressure higher than or equal to the atmospheric pressure. Alternatively, (ii) the sensor may detect the temperature of the working fluid, and the first threshold value may be a temperature higher than or equal to the boiling point of the working fluid at atmospheric pressure, or (iii) the sensor may detect the temperature of a cooling medium to be heat-exchanged with the working fluid in the condenser, and the first threshold value may be a temperature higher than or equal to the boiling point of the working fluid at atmospheric pressure.

Each of (i), (ii), and (iii) of the eleventh aspect is suitable for preventing the pressure of the working fluid from becoming negative.

According to a twelfth aspect of the present disclosure, for example, in the control method according to the tenth aspect or the eleventh aspect, a circulation circuit in which the pump, the evaporator, the expander, and the condenser are provided in this order may be provided in the Rankine cycle apparatus, and the sensor may detect the pressure of the working fluid in a portion of the circulation circuit that is downstream of the expander and upstream of the pump.

The twelfth aspect is suitable for preventing the pressure of the working fluid from becoming negative.

According to a thirteenth aspect of the present disclosure, for example, in the control method according to any one of the tenth to twelfth aspects, the Rankine cycle apparatus may include a circulation circuit in which the pump, the evaporator, the expander, and the condenser are provided in this order and a bypass circuit configured to connect a portion of the circulation circuit that is downstream of the evaporator and upstream of the expander to a portion of the circulation circuit that is downstream of the expander and upstream of the condenser, and the working fluid may pass through the bypass circuit in the first circulation.

In the first circulation according to the thirteenth aspect, the working fluid can circulate by bypassing the expander by using the bypass circuit. This configuration can smoothly circulate the working fluid in the first circulation.

According to the fourteenth aspect of the present disclosure, for example, in the control method according to the thirteenth aspect, a valve may be provided in the bypass circuit, and an opening degree of the valve of the bypass circuit may be set to 50% or more and 100% or less in the first circulation.

According to the fourteenth aspect, the opening degree of the valve in the bypass circuit is set to 50% or more and 100% or less in the first circulation. If the opening degree is set in this way, the working fluid can be easily circulated in the first circulation.

According to a fifteenth aspect of the present disclosure, for example, in the control method according to any one of the tenth to fourteenth aspects, the working fluid may be heated by the evaporator and/or a heater in the first circulation.

The evaporator and the heater are specific examples of devices that heat the working fluid.

According to a sixteenth aspect of the present disclosure, for example, in the control method according to any one of the tenth to fifteenth aspects, the working fluid may be heated by the evaporator in the first circulation, and the method may further include starting heating the working fluid by the heater if the detected value is less than a second threshold value and an elapsed time from the start of the first circulation is greater than or equal to a threshold time.

According to the sixteenth aspect, even if the risk of the pressure of the working fluid becoming negative in the first circulation cannot be sufficiently prevented, the above-mentioned risk can be prevented by using the heater.

According to a seventeenth aspect of the present disclosure, for example, the control method according to any one of the tenth to sixteenth aspects may further include stopping the driving of the pump if the detected value is greater than or equal to a second threshold value.

According to the seventeenth aspect, unnecessary power consumption of the pump can be avoided.

According to an eighteenth aspect of the present disclosure, for example, in the control method according to any one of the tenth to seventeenth aspects, the boiling point of the working fluid at atmospheric pressure is 0° C. or higher and 50° C. or lower.

When the boiling point of the working fluid is high to the extent specified in in the eighteenth aspect, the pressure of the working fluid is likely to be negative. Therefore, in this case, the technique for preventing the pressure of the working fluid from becoming negative is likely to exert its effect.

According to a nineteenth aspect of the present disclosure, a Rankine cycle apparatus includes a sensor, a pump, an evaporator, an expander, a condenser, and a fluid circuit through which a working fluid to flows. The fluid circuit includes a circulation circuit in which the pump, the evaporator, the expander, and the condenser are provided in this order. The sensor is configured to detect one of (I) the pressure of the working fluid, (II) the temperature of the working fluid, and (III) the temperature of a cooling medium to be heat-exchanged with the working fluid in the condenser. First control is started if a detected value of the sensor is less than a first threshold value, the first control is control to cause the pump to circulate the working fluid through the evaporator and/or a heater.

According to a twentieth aspect of the present disclosure, a method is provided for controlling a Rankine cycle apparatus configured to circulate a working fluid through a pump, an evaporator, an expander, and a condenser in this order. The method includes detecting, by using a sensor, one of (I) the pressure of the working fluid, (II) the temperature of the working fluid, and (III) the temperature of a cooling medium to be heat-exchanged with the working fluid in the condenser and starting first circulation to circulate the working fluid through the evaporator and/or a heater by using the pump if a detected value of the sensor is less than a first threshold value.

In the following embodiments, the term “circuit” may be used. As can be understood from the drawings and the like, the term “circuit” does not necessarily mean a closed path and can be used interchangeably with the term “flow passage” as appropriate.

Embodiments are described below with reference to the accompanying drawings. The present disclosure is not limited to the embodiments.

First Embodiment

FIG. 1 is a configuration diagram of a Rankine cycle apparatus 21 according to the first embodiment.

The Rankine cycle apparatus 21 is provided with a fluid circuit 14. In the fluid circuit 14, working fluid flows. The fluid circuit 14 includes a circulation circuit 15 and a bypass circuit 16.

The type of working fluid is not limited to any particular type. The boiling point of the working fluid at atmospheric pressure is, for example, 0° C. or higher and 50° C. or lower. As used herein, the atmospheric pressure is one atmosphere. A specific example of the working fluid is a hydrofluoroolefin (HFO)-based working fluid. Note that the HFO-based working fluid here is a working fluid containing HFO. The content percentage of HFO in the working fluid is, for example, 50% or more by mass and may be 80% or more by mass. More specifically, as the working fluid, a mixed fluid of HFO1336mzz(Z), HFO1336mzz(E), HFO1336mzz(Z), and HFO1336mzz(E) can be adopted. The working fluid containing HFO may be a mixed fluid or a pure working fluid. A widely used fluid that does not contain HFO can be used as the working fluid.

The fluid circuit 14 is configured using a plurality of pipes. Hereinafter, a plurality of pipes may be collectively referred to as a pipe unit.

A Rankine cycle is formed in the Rankine cycle apparatus 21. More specifically, the Rankine cycle apparatus 21 forms an organic Rankine cycle (ORC).

The Rankine cycle apparatus 21 includes a pump 1, an evaporator 2, an expander 3, and a condenser 4. In the circulation circuit 15, the pump 1, the evaporator 2, the expander 3, and the condenser 4 are provided in this order. The pump 1, the evaporator 2, the expander 3, and the condenser 4 are connected using a plurality of pipes.

The Rankine cycle apparatus 21 includes a reheater 6. In the reheater 6, heat is exchanged between the working fluids flowing in different portions of the circulation circuit 15.

The bypass circuit 16 connects a portion of the circulation circuit 15 that is downstream of the evaporator 2 and upstream of the expander 3 to a portion of the circulation circuit 15 that is downstream of the expander 3 and upstream of the condenser 4.

The Rankine cycle apparatus 21 includes a valve 5. The valve 5 is provided in the bypass circuit 16. Hereinafter, the valve 5 is also referred to as a bypass valve 5. According to the present embodiment, the bypass valve 5 is a flow rate regulating valve. Note that the flow rate regulating valve is a valve capable of having not only opening degrees of 0% and 100% but an opening degree varied between 0% and 100%.

The Rankine cycle apparatus 21 includes an electric generator 18. The electric generator 18 is connected to the expander 3.

The operations performed by the pump 1, the evaporator 2, the expander 3, the condenser 4, the bypass valve 5, the reheater 6, and the electric generator 18 when the Rankine cycle apparatus 21 is performing the power generation operation are described below. Note that the power generation operation performed by the Rankine cycle apparatus 21 refers to an operation performed by the electric generator 18 to generate power.

The pump 1 conveys a working fluid.

The evaporator 2 evaporates the working fluid. More specifically, the evaporator 2 recovers the heat of a heating medium and evaporates the working fluid. According to the present embodiment, the heating medium is a heat source gas. More specifically, according to the present embodiment, the heating medium is exhaust gas from a heat source, such as factory equipment. The evaporator 2 is composed of, for example, a fin-and-tube heat exchanger.

The expander 3 expands the working fluid. More specifically, the expander 3 expands the working fluid that has become high temperature steam in the evaporator 2.

The condenser 4 condenses the working fluid expanded by the expander 3. More specifically, the condenser 4 condenses the working fluid by removing the heat of the working fluid by the cooling medium. According to the present embodiment, the cooling medium is gas and, in particular; air in the atmosphere. However, the cooling medium may be a liquid, such as water. According to the present embodiment, the condenser 4 includes a fan 7. The condenser 4 uses the fan 7 to condense the working fluid. However, the fan 7 is not essential. The condenser 4 is composed of, for example, a fin-and-tube heat exchanger, a plate heat exchanger, or a double-tube heat exchanger. In one specific example, the cooling medium is air, the condenser 4 is a fin and tube heat exchanger, and the condenser 4 includes a fan 7. In another specific example, the cooling medium is water, the condenser 4 is a plate heat exchanger or a double tube heat exchanger, and the condenser 4 does not include the fan 7.

The bypass valve 5 regulates the flow rate of the working fluid flowing through the expander 3 and the flow rate of the working fluid flowing through the bypass circuit 16. More specifically, these flow rates are regulated by controlling the opening degree of the bypass valve 5.

The reheater 6 has a first portion 6 a that is downstream of the pump 1 and upstream of the evaporator 2 in the circulation circuit 15 and a second portion 6 b that is downstream of the expander 3 and upstream of the condenser 4 in the circulation circuit 15. In the reheater 6, heat is exchanged between the working fluids flowing through the two portions. Due to this heat exchange, the temperature of the working fluid flowing through the first portion 6 a increases, while the temperature of the working fluid flowing through the second portion 6 b decreases.

The electric generator 18 generates electric power by the rotation torque of the expander 3.

Furthermore, the Rankine cycle apparatus 21 includes a first pressure sensor 8 a, a second pressure sensor 8 b, a first temperature sensor 9 a, a second temperature sensor 9 b, a third temperature sensor 9 c, and a fourth temperature sensor 9 d.

The first pressure sensor 8 a detects the pressure of the working fluid in the first circuit 15 a. The second pressure sensor 8 b detects the pressure of the working fluid in the second circuit 15 b. Note that the first circuit 15 a is a portion of the circulation circuit 15 that is downstream of the pump 1 and upstream of the expander 3. The second circuit 15 b is a portion of the circulation circuit 15 that is downstream of the expander 3 and upstream of the pump 1.

According to the present embodiment, the first pressure sensor 8 a is provided in the first circuit 15 a. More specifically, according to the present embodiment, the first pressure sensor 8 a is provided downstream of the evaporator 2 and upstream of the expander 3 in the circulation circuit 15. However, the first pressure sensor 8 a may be provided downstream of the pump 1 and upstream of the evaporator 2 in the circulation circuit 15. The first pressure sensor 8 a detects the pressure on the high pressure side of the Rankine cycle of the Rankine cycle apparatus 21. The first pressure sensor 8 a can be referred to as a high pressure sensor 8 a.

The first pressure sensor 8 a may be provided in a portion of the bypass circuit 16 that is upstream of the bypass valve 5. The first pressure sensor 8 a provided upstream of the bypass valve 5 in the bypass circuit 16 can also detect the pressure of the working fluid in the first circuit 15 a.

According to the present embodiment, the second pressure sensor 8 b is provided in the second circuit 15 b. More specifically, according to the present embodiment, the second pressure sensor 8 b is provided in a portion of the circulation circuit 15 that is downstream of the condenser 4 and upstream of the pump 1. As described below, the portion corresponds to a third circuit 15 c. However, the second pressure sensor Sb may be provided in a portion of the circulation circuit 15 that is downstream of the expander 3 and upstream of the condenser 4. The second pressure sensor 8 b detects the pressure on the low pressure side of the Rankine cycle of the Rankine cycle apparatus 21. The second pressure sensor Sb can be referred to as a low pressure sensor 8 b.

The second pressure sensor 8 b may be provided in a portion of the bypass circuit 16 that is downstream of the bypass valve 5. The second pressure sensor 8 b provided in the portion of the bypass circuit 16 that is downstream of the bypass valve 5 can also detect the pressure of the working fluid in the second circuit 15 b.

The first temperature sensor 9 a is provided in a portion of the circulation circuit 15 that is downstream of the evaporator 2 and upstream of the expander 3. The first temperature sensor 9 a detects the temperature of the working fluid in this portion. By using the first temperature sensor 9 a, the temperature of the working fluid at the inlet of the expander 3 can be sensed. The first temperature sensor 9 a can be referred to as an expander inlet temperature sensor 9 a.

The second temperature sensor 9 b is provided in the third circuit 15 c. Note that the third circuit 15 c is a portion of the circulation circuit 15 that is downstream of the condenser 4 and upstream of the pump 1, The second temperature sensor 9 b detects the temperature of the working fluid in the third circuit 15 c. By using the second temperature sensor 9 b, the temperature of the working fluid at the outlet of the condenser 4 can be sensed. The second temperature sensor 9 b can be referred to as a condenser outlet temperature sensor 9 b.

The third temperature sensor 9 c is disposed in a portion of the condenser 4 at which the cooling medium is sucked. The third temperature sensor 9 c detects the temperature of the cooling medium. According to the present embodiment, the cooling medium is air in the atmosphere, and the third temperature sensor 9 c detects the outside air temperature. According to the present embodiment, the third temperature sensor 9 c can be referred to as an outside air temperature sensor 9 c.

The fourth temperature sensor 9 d detects the temperature of the heating medium sucked into the evaporator 2. As described above, according to the present embodiment, the heating medium is the heat source gas. According to the present embodiment, the fourth temperature sensor 9 d can be referred to as a heat source gas temperature sensor 9 d.

Furthermore, the Rankine cycle apparatus 21 includes a controller 19. The controller 19 controls the constituent elements of the Rankine cycle apparatus 21.

The pressure of the working fluid in the fluid circuit 14 is described below.

When the Rankine cycle apparatus 21 performs a power generation operation, the evaporator 2 recovers the heat of the heating medium, and the heat heats the working fluid. Thereafter, the heated working fluid flows through the fluid circuit 14 by the pump 1. In this manner, the pressure of the working fluid in the fluid circuit 14 is maintained at a positive pressure. Note that the positive pressure is a pressure higher than the atmospheric pressure.

However, when the Rankine cycle apparatus 21 is stopped and the pump 1 is stopped, the working fluid does not flow in the fluid circuit 14. In this case, even if the temperature of the heating medium supplied to the evaporator 2 is high, the pressure of the working fluid in the fluid circuit 14 may be influenced by the outside air temperature and, thus, the pressure of the working fluid may be close to the saturation pressure of the working fluid at the outside air temperature.

In one specific example, the pump 1, the expander 3, and the condenser 4 are housed in one enclosure, and the enclosure and the evaporator 2 are separated by 5 m or more. When the separation distance is that large, the pressure of the working fluid in the fluid circuit 14 existing in the enclosure is more susceptible to the outside air temperature than to the temperature of the heating medium and, thus, the pressure of the working fluid tends to be close to the saturation pressure of the working fluid at the outside air temperature.

The case is discussed below where a working fluid having a high saturation temperature at atmospheric pressure is used. In this case, if the outside air temperature is lower than the boiling point of the working fluid, the working fluid may have a negative pressure. Note that the saturation temperature refers to the boiling point. Negative pressure refers to pressure lower than the atmospheric pressure.

With reference to FIG. 2, a description is given of how the pressure of the working fluid becomes negative under the influence of the outside air temperature when the temperature of the working fluid is in substantial equilibrium in each of the portions of the fluid circuit 14 and the outside air temperature is lower than the boiling point of the working fluid. Note that, typically, immediately after the pump 1 is stopped, the temperature of the working fluid varies from portion to portion of the fluid circuit 14. After sufficient time has elapsed since the stoppage of the pump 1, the temperature of the working fluid can reach substantial equilibrium at each of the portions of the fluid circuit 14. FIG. 2 is a state diagram of the working fluid. Note that the melting and sublimation curves are not given in FIG. 2.

In state A, the working fluid is in a gas-liquid two-phase state. The pressure of the working fluid is positive. The temperature of the working fluid is above its boiling point at atmospheric pressure.

If the outside air temperature varies, the temperature of the working fluid also varies under the influence of the outside air temperature. In particular, the temperature of the working fluid inside the condenser 4 becomes substantially the same as the outside air temperature when the temperature is in thermal equilibrium. For example, if the outside air temperature decreases, the temperature of the working fluid also decreases. As the temperature of the working fluid decreases, the pressure of the working fluid decreases along the vapor pressure curve. More specifically, in this example, the state of the working fluid changes from state A to state C via state B.

In state B, the working fluid is in a gas-liquid two-phase state. The pressure of the working fluid is the atmospheric pressure. The temperature of the working fluid is the boiling point at atmospheric pressure.

In state C, the working fluid is in a gas-liquid two-phase state. The temperature of the working fluid is the outside air temperature. The pressure of the working fluid is negative.

In FIG. 2, the pressure of the working fluid is negative even in a state between state B and state C in the vapor pressure curve. As can be understood from this, even if the temperature of the working fluid does not drop to the outside air temperature, the pressure of the working fluid can be negative.

Note that FIG. 2 is for illustrative purposes only and should not be used to interpret the embodiments in a limiting manner. For example, the curves of the working fluid state diagram are not limited to the shapes in FIG. 2, In addition, the pressure of the working fluid that is not in the gas-liquid two-phase state may be negative.

More specifically, suppose that HFO1336mzz(Z) is used as the working fluid. The boiling point of HFO1336mzz(Z) is 33° C. Therefore, in this case, if the pump 1 is stopped when the outside air temperature is lower than 33° C., the pressure of the working fluid may become negative.

Alternatively, suppose that HFO1336mzz(E) is used as the working fluid. The boiling point of HFO1336mzz(E) is 8° C. Therefore, in this case, if the pump 1 is stopped when the outside air temperature is lower than 8° C., the pressure of the working fluid may become negative.

In the pipe unit, a gap may occur at a welding point, a screw connection portion, or the like. The gap may be caused by poor construction, such as poor welding, insufficient tightening torque at the time of screw connection, loosening of the screw connection portion due to vibration during operation, or aging deterioration. When the pressure of the working fluid is positive and if a gap occurs in the pipe unit, the working fluid leaks from the inside of the pipe unit to the outside. In this case, if the pipe unit is repaired so that the gap is eliminated and the working fluid is refilled, the Rankine cycle apparatus 21 can be returned to the state before the working fluid leaks. In addition, if the working fluid leaks from the pipe unit, a specific symptom appears that suggests a malfunction of the Rankine cycle apparatus, such as the inability of the pump 1 to circulate the working fluid due to lack of working fluid. For this reason, the leakage of the working fluid from the pipe unit is easily recognized. Therefore, the repair of the pipe unit and the refilling of the working fluid can be carried out relatively soon from the occurrence of the leakage of the working fluid.

In contrast, when the pressure of the working fluid is a negative pressure, and when a gap occurs in the pipe unit, air, moisture, or the like in the atmosphere may be mixed into the pipe unit. If such mixing occurs, the working fluid or lubricating oil may be hydrolyzed. If the lubricating oil is hydrolyzed, the lubricity of the sliding parts of the components of the equipment, such as the pump 1 and the expander 3, deteriorates, which may cause equipment failure. It is not always easy to quickly recognize the entry of air, moisture, or the like into the pipe unit. For this reason, when it is noticed that a failure has occurred, the level of failure may be high. Thus, at the time the failure is discovered, the failure of the equipment has progressed to a serious level, and it is difficult to return to the original state even if the gap is repaired and the working fluid is refilled.

To avoid the above-mentioned failure caused by the negative pressure of the working fluid when a gap is formed in the pipe unit, it is conceivable to eliminate the generation of the gap itself in the pipe unit. However, it is not always easy to completely eliminate the generation of a gap.

More specifically, as described above, the fluid circuit 14 may include a welding point, a screw connection portion, and the like. It is not easy to completely eliminate the generation of gaps at a welded point, a screw connection portion and the like.

By adopting an installation technique in which the Rankine cycle apparatus 21 is built in a factory equipped with manufacturing equipment and the finished product is moved to the installation site, that is, by eliminating the welding work at the installation site of the Rankine cycle apparatus 21, the probability of the occurrence of gaps at the welding point can be reduced. However, such an installation technique is not adopted at all times. If the heat source that supplies the heat source gas to the evaporator 2 is equipment fixed to the land, the Rankine cycle apparatus 21 may be installed at the site where the equipment is located. When welding is carried out on site, it is not always easy to completely prevent the generation of gap at a welding point.

Furthermore, even if no problem occurs when the Rankine cycle apparatus 21 is installed, a gap may occur in the pipe unit due to vibration during operation of the Rankine cycle apparatus 21. It is not always easy to completely avoid the generation of gaps during operation.

Accordingly, to avoid the occurrence of the above-mentioned failure caused by the occurrence of negative pressure of the working fluid when a gap is formed in the pipe unit, the present inventors have studied to prevent the occurrence of negative pressure in the pipe unit of the Rankine cycle apparatus 21.

According to the study by the present inventors, if the pressure of the working fluid is low, the occurrence of negative pressure can be prevented by driving the pump 1 in a stopped state. More specifically, by driving the pump 1, the working fluid can be heated in the evaporator 2 while the working fluid is flowing in the fluid circuit 14 and, thus, the pressure of the working fluid can be maintained positive. The present inventors have further studied and have conceived the idea of the control described below.

Hereinafter, the control of the Rankine cycle apparatus 21 is described with reference to a flowchart illustrated in FIG. 3. In the following description, it is assumed that the bypass valve 5 is closed before the start of the processing described in the flowchart.

In step S1, the controller 19 determines whether the pump 1 is stopped and the pressure detected by the second pressure sensor 8 b is lower than a first threshold pressure Pth1. If the pump 1 is stopped and the pressure detected by the second pressure sensor 8 b is lower than the first threshold pressure Pth1, the processing proceeds to step S2. If the pump 1 is driven and/or if the pressure detected by the second pressure sensor 8 b is higher than or equal to the first threshold pressure Pth1 step 31 is performed again. The condition “the pump 1 is driven and/or the pressure detected by the second pressure sensor 8 b is higher than or equal to the first threshold pressure Pth1” means that at least one of the condition “the pump 1 is driven” and the condition “the pressure detected by the second pressure sensor 8 b is higher than or equal to the first threshold pressure Pth1” is satisfied.

The first threshold pressure Pth1 can be a pressure higher than or equal to the atmospheric pressure. The first threshold pressure Pth1 can be a pressure lower than or equal to the pressure detected by the second pressure sensor 8 b during the power generation operation and, more specifically, a pressure lower than the detected pressure. In this context, the term “detected pressure of the second pressure sensor 8 b during the power generation operation” refers to the detected value of the pressure in the steady state rather than the transient state. The first threshold pressure Pth1 is, for example, 0.01 MPa or higher and 0.2 MPa or lower. In one specific example; the first threshold pressure Pth1 is 0.05 MPa.

In this example, the elapsed time during which the condition in step S1 is satisfied is counted from when the processing first proceeds from step S1 to step S2. Hereinafter, the elapsed time is also referred to as a standby time.

In step S2, the controller 19 determines whether the standby time is greater than or equal to a threshold standby time Twth. If the standby time is greater than or equal to the threshold standby time Twth, the processing proceeds to step S3. If the standby time is less than the threshold standby time Twth, the processing proceeds to step S1.

The threshold standby time Twth is, for example, 0.1 minutes or greater and 5 minutes or less. A specific example of the threshold standby time Twth is 1 minute.

In step S3, the controller 19 increases the opening degree of the bypass valve 5. In addition, in step S3, the controller 19 starts driving the pump 1. After step S3, the processing proceeds to step S4.

After step S3 is performed, the working fluid circulates through the pump 1, the evaporator 2, the bypass valve 5, and the condenser 4 in this order. In the evaporator 2, the heat of the heating medium supplied to the evaporator 2 is recovered, and the recovered heat heats the working fluid. This heating increases the pressure of the working fluid.

As described above, the heating medium supplied to the evaporator 2 can be a heat source gas. In one specific example, the heat source gas is exhaust gas from a facility having a large heat capacity, such as a drying furnace or a blast furnace. In this case, even if the facility is stopped operating, the temperature of the exhaust gas does not immediately drop to near the outside air temperature. For this reason, the temperatures of the evaporator 2 and its ambient air temperature remain higher than the outside air temperature for a certain length of time after the operation is stopped. Therefore, by driving the pump 1, the evaporator 2 can heat the working fluid. Thus, the pressure of the working fluid can be increased.

In step 33, the bypass valve 5 may be fully opened. If the opening degree of the bypass valve 5 is not zero before step 33 is performed (for example, if the opening degree is 50% or more), the opening degree of the bypass valve 5 does not necessarily have to be increased in step S3. In some cases, the opening degree of the bypass valve 5 may be maintained at zero before and after step S3.

In step 33, the rotational speed of the pump 1 is set to, for example, 100 rpm or more and 5000 rpm or less. In one specific example, in step 33, the rotational speed of the pump 1 is set to 1000 rpm. However, since the operating speed range varies depending on the specification of the pump, the rotational speed to be set is not limited to the above example.

The operation of step S3 can be considered as control for preventing negative pressure. The control in step S3 can be referred to as first negative pressure prevention control.

The second pressure sensor 8 b is provided in the second circuit 15 b. The pressure of the working fluid in the second circuit 15 b tends to be negative. For this reason, if the negative pressure prevention control is performed on the basis of the detected value of the second pressure sensor 8 b, the effect of preventing the negative pressure of the working fluid can be easily obtained. More specifically, the second pressure sensor 8 b is provided in the third circuit 15 c.

In step S4, the controller 19 determines whether the pressure detected by the second pressure sensor 8 b is higher than or equal to a second threshold pressure Pth2. If the pressure detected by the second pressure sensor 8 b is higher than or equal to the second threshold pressure Pth2, the processing proceeds to step S5. If the pressure detected by the second pressure sensor 8 b is lower than the second threshold pressure Pth2, step 34 is performed again.

The second threshold pressure Pth2 can be a pressure higher than or equal to the atmospheric pressure and, in particular, a pressure higher than the atmospheric pressure. The second threshold pressure Pth2 can be a pressure lower than or equal to the detected pressure of the second pressure sensor 8 b during the power generation operation and, in particular, a pressure lower than the detected pressure. According to the present embodiment, the second threshold pressure Pth2 is higher than the first threshold pressure Pth1. The second threshold pressure Pth2 is, for example, 0.01 MPa or higher and 0.2 MPa or lower. In one specific example, the second threshold pressure Pth2 is 0.15 MPa.

In step S5, the controller 19 stops the pump 1, resulting in end of the negative pressure prevention control in step S3.

After the control based on the flowchart in FIG. 3 ends, the control may be restarted. For example, the control may be restarted after a predetermined period has elapsed from the end of the control based on the flowchart in FIG. 3. This also applies to the control of the embodiments described later.

Some other embodiments are described below. In the following description, the same reference numerals are used for elements that are common to both the above embodiment and the following embodiment, and description of the elements is not repeated as needed. The same is true for the flowcharts. The descriptions of each embodiment may be applied to each other unless they are technically inconsistent. The embodiments may be combined with each other unless they are technically inconsistent.

Second Embodiment

FIG. 4 is a configuration diagram of a Rankine cycle apparatus 22 according to the second embodiment.

The Rankine cycle apparatus 22 includes a heater 10. The heater 10 is, for example, a resistive heater.

The heater 10 is provided in the fluid circuit 14. The heater 10 heats the working fluid flowing through the fluid circuit 14. In the example illustrated in FIG. 4, the heater 10 is provided in the circulation circuit 15.

More specifically, the heater 10 is provided in a portion of the circulation circuit 15 that is downstream of the pump 1 and upstream of the condenser 4. That is, the heater 10 is provided in a portion of the circulation circuit 15 other than a portion downstream of the condenser 4 and upstream of the pump 1. This configuration is less likely to cause a situation in which the working fluid flowing into the pump 1 due to the heating by the heater 10 is in a gas-liquid two-phase state, and a problem in conveyance of the working fluid by the pump 1 is less likely to occur.

More specifically, the heater 10 is provided in a portion of the circulation circuit 15 that is downstream of the pump 1 and upstream of the evaporator 2. More specifically, the heater 10 is provided in a portion of the circulation circuit 15 that is downstream of the pump 1 and upstream of the reheater 6.

In one example, the heater 10 has a linear shape. The heater 10 is in tight contact with the pipe in the circulation circuit 15. The length direction of the pipe and the length direction of the heater 10 coincide.

In another example, the heater 10 has a strip shape. The heater 10 is wound around the outer wall of the pipe in the circulation circuit 15.

The controller 19 controls the heater 10. According to the present embodiment, the controller 19 controls energizing of the heater 10. When the heater 10 is energized, the heater 10 generates heat. When the heater 10 is not energized, the heater 10 does not generate heat.

The control of the Rankine cycle apparatus 22 is described with reference to the flowchart illustrated in FIG. 5.

According to the second embodiment, the elapsed time from the start of driving the pump 1 in step S3 is counted. Hereinafter, this elapsed time is referred to as a pump operation time.

According to the second embodiment, if, in step S4, the pressure detected by the second pressure sensor 8 b is lower than the second threshold pressure Pth2, the processing proceeds to step S6.

In step S6, the controller 19 determines whether the pump operation time is greater than or equal to a threshold time Tth. If the pump operating time is greater than or equal to the threshold time Tth, the processing proceeds to step S7. If the pump operating time is less than the threshold time Tth, the processing proceeds to step S4.

The threshold time Tth is, for example, 1 minute or greater and 10 minutes or less. A specific example of the threshold time Tth is 5 minutes.

In step S7, the controller 19 starts energizing the heater 10. The heating of the working fluid by the heater 10 is started by the start of energization of the heater 10 in step S7. The processing proceeds to step S8 after step S7 is performed.

Suppose that the heat source is factory equipment which has been stopped for a long period of time. In this case, since the factory equipment is cooled by the ambient air, the difference between the temperature of the heat source gas and the outside air temperature may be small. If the difference is small, the working fluid is likely not to be sufficiently heated in the evaporator 2 even if the pump 1 is driven in step 3. In contrast, according to the present embodiment, the working fluid is heated by the heater 10 in step S7. As a result, the pressure of the working fluid can be sufficiently increased.

The operation in step 37 can be considered as control for preventing negative pressure. The control in step S7 can be referred to as second negative pressure prevention control.

In step S8, the controller 19 determines whether the pressure detected by the second pressure sensor 8 b is higher than or equal to the second threshold pressure Pth2. If the pressure detected by the second pressure sensor 8 b is higher than or equal to the second threshold pressure Pth2, the processing proceeds to step 39. If the pressure detected by the second pressure sensor 8 b is lower than the second threshold pressure Pth2, step S8 is performed again.

In step S9, the controller 19 terminates energization of the heater 10. The termination of energization of the heater 10 in step 39 terminates heating of the working fluid by the heater 10. As a result, the second negative pressure prevention control in step S7 ends.

In step 39, the controller 19 may stop energization of the heater 10 and, at the same time, stop the pump 1.

Third Embodiment

FIG. 6 is a configuration diagram of a Rankine cycle apparatus 23 according to the third embodiment.

In the Rankine cycle apparatus 23, the fluid circuit 14 includes a shortcut circuit 17.

The Rankine cycle apparatus 23 includes a valve 11. The valve 11 is provided in the shortcut circuit 17. Hereinafter, the valve 11 is referred to as a shortcut valve 11. According to the present embodiment, the shortcut valve 11 is a flow rate regulating valve.

The control based on the flowchart in FIG. 5 can be applied to the third embodiment. According to the third embodiment, when step S7 in FIG. 5 is performed, the controller 19 increases the opening degree of the shortcut valve 11, As a result, the working fluid circulates through the pump 1, the shortcut valve 11, and the condenser 4 in this order. During the circulation, the working fluid also passes through the heater 10. The working fluid is heated by the heater 10. In this manner, the pressure of the working fluid can be increased.

Since the circulation path of the working fluid passing through the pump 1, the shortcut valve 11, and the condenser 4 in this order is shorter than the circulation path of the working fluid passing through the pump 1, the evaporator 2, the bypass valve 5, and the condenser 4 in this order. For this reason, the working fluid can be circulated quickly by the pump 1. Such a short circulation path can help the heater 10 increase the pressure of the working fluid.

The shortcut valve 11 may be fully opened when step S7 in FIG. 5 is performed. If the opening degree of the shortcut valve 11 is not zero before step S7 is performed (for example, if the opening degree is 50% or greater), the opening degree of the shortcut valve 11 does not necessarily have to be increased when step S7 is performed. In some cases, the opening degree of the shortcut valve 11 may be maintained at zero before and after step S7 is performed.

Fourth Embodiment

The control according to the fourth embodiment is described with reference to FIG. 7. The control according to the fourth embodiment can be performed by using, for example, the Rankine cycle apparatus 22 according to the second embodiment.

As illustrated in FIG. 7, according to the fourth embodiment, if, in step S2, the standby time is greater than or equal to the threshold standby time Twth, the processing proceeds to step 310.

In step S10, the controller 19 increases the opening degree of the bypass valve 5. In step 310, the controller 19 starts energizing the heater 10. When energization of the heater 10 is started in step S10, heating of the working fluid by the heater 10 is started. In addition, in step S10, the controller 19 starts driving the pump 1, After step S10 is performed, the processing proceeds to step S4.

After step 310 is performed, the working fluid circulates through the pump 1, the heater 10, the evaporator 2, the bypass valve 5, and the condenser 4 in this order. The working fluid is heated by the heater 10. In addition, the working fluid can be heated in the evaporator 2.

In step S10, the bypass valve 5 may be fully opened. If the opening degree of the bypass valve 5 is not zero before step S10 is performed (for example, if the opening degree is 50% or greater), the opening degree of the bypass valve 5 need not be increased in step S10. In some cases, the opening degree of the bypass valve 5 may be maintained at zero before and after step 310 is performed.

In some cases, the controller 19 may energize the heater 10 before step S10 is performed. For example, the controller 19 may start energizing the heater 10 when the standby time reaches the first threshold time, and the controller 19 may start driving the pump 1 when the standby time reaches the threshold standby time Twth. Note that the first threshold time is less than the threshold standby time Twth.

Alternatively, the controller 19 may drive the pump 1 before step S10 is performed. For example, the controller 19 may start driving the pump 1 when the standby time reaches the second threshold time, and the controller 19 may start energizing the heater 10 when the standby time reaches the threshold standby time Twth. Note that the second threshold time is less than the threshold standby time Twth.

In step S10, the rotational speed of the pump 1 is set to, for example, 100 rpm or more and 5000 rpm or less. In one specific example, in step S10, the rotational speed of the pump 1 is set to 1000 rpm. However, since the operating rotational speed range varies depending on the pump specification, the rotational speed to be set is not limited to the above example.

The operation in step S10 can be considered as control for preventing negative pressure. The control in step 310 can be referred to as third negative pressure prevention control.

According to the fourth embodiment, if, in step S4, the pressure detected by the second pressure sensor 8 b is higher than or equal to the second threshold pressure Pth2, the processing proceeds to step S11.

In step S11, the controller 19 terminates energization of the heater 10. In step S11, the controller 19 stops the pump 1. After step 311 is performed, the negative pressure prevention control in step S10 ends.

When the heat source is factory equipment which has been stopped for a long period of time, there may be substantially no difference between the temperature of the heat source gas and the outside air temperature. In this case, even if the pump 1 is driven, the working fluid is not substantially heated in the evaporator 2. Therefore, the evaporator 2 does not substantially contribute to an increase in pressure of the working fluid.

In contrast, according to the fourth embodiment, even when the evaporator 2 does not contribute to an increase in the temperature and the pressure of the working fluid, the increase in the temperature and the pressure of the working fluid can be increased by the heater 10.

Fifth Embodiment

The control according to the fourth embodiment can be performed by using the Rankine cycle apparatus 23 according to the third embodiment. An embodiment in which the control according to the fourth embodiment is performed by using the Rankine cycle 23 apparatus according to the third embodiment is referred to as a fifth embodiment.

According to the fifth embodiment, when step S10 in FIG. 7 is performed, the opening degree of the shortcut valve 11 may be increased or may be fully opened. If the opening degree of the shortcut valve 11 is not zero before step S10 is performed (for example, if the opening degree is 50% or greater), the opening degree of the shortcut valve 11 need not be increased when step S10 is performed. In some cases, the opening degree of the shortcut valve 11 may be maintained at zero before and after step S10 is performed.

According to the fifth embodiment, after step 310 in FIG. 7 is performed, a configuration in which the opening degree of the shortcut valve 11 is non-zero and the opening degree of the bypass valve 5 is zero can be adopted. In this configuration, after step S10 is performed, the working fluid circulates through the pump 1, the heater 10, the shortcut valve 11, and the condenser 4 in this order. The working fluid is heated in the heater 10. The heating increases the pressure of the working fluid.

Technology Applicable to First to Fifth Embodiments

In the above-described examples, in step S1 of first to fifth embodiments, it is determined whether the pressure detected by the second pressure sensor 8 b is lower than the first threshold pressure Pth1, However, instead of making the determination, it may be determined whether the temperature detected by the second temperature sensor 9 b is lower than the first threshold temperature. Alternatively, instead of making the determination, it may be determined whether the temperature detected by the third temperature sensor 9 c is lower than the second threshold temperature. This is because the temperature detected by the second temperature sensor 9 b and the temperature detected by the third temperature sensor 9 c can be effective indices for preventing the pressure of the working fluid from becoming negative.

The pressure detected by the second pressure sensor 8 b used in step S1 of the first to fifth embodiments is the pressure detected on the low pressure side of the Rankine cycle. In step S1 the detected pressure on the high pressure side of the Rankine cycle may be used. More specifically, in step S1, it may be determined whether the pressure detected by the first pressure sensor 8 a is lower than a third threshold pressure. This is because the detected pressure on the high pressure side can also be an effective index for preventing the pressure of the working fluid from becoming negative. When the pump 1 is stopped, the pressure on the low pressure side and the pressure on the high pressure side may be close to each other.

Furthermore, in step S1 the detected temperature on the high pressure side of the Rankine cycle may be used. More specifically, in step S1, it may be determined whether the temperature detected by the first temperature sensor 9 a is lower than the third threshold temperature. This is because the detected temperature on the high pressure side can also be an effective index for preventing the pressure of the working fluid from becoming negative. When the pump 1 is stopped, the pressure on the low pressure side and the pressure on the high pressure side may be close to each other.

More specifically, in step S1 according to one modification, the controller 19 determines whether the pump 1 is stopped and the temperature detected by the second temperature sensor 9 b is lower than the first threshold temperature. If the pump 1 is stopped and the temperature detected by the second temperature sensor 9 b is lower than the first threshold temperature, the processing proceeds to step S2. If the pump 1 is driven and/or if the temperature detected by the second temperature sensor 9 b is higher than or equal to the first threshold temperature, step S1 is performed again. The first threshold temperature can be a temperature higher than or equal to the boiling point of the working fluid at atmospheric pressure and, more specifically, a temperature higher than the boiling point. The first threshold temperature can be a temperature lower than or equal to the detected temperature of the second temperature sensor 9 b during the power generation operation and, more specifically, a temperature lower than the detected temperature. The first threshold temperature is, for example, a value obtained by adding a margin to the boiling point of the working fluid at atmospheric pressure. The margin is, for example, 0° C. or higher and 5° C. or lower.

In step S1 according to one modification, the controller 19 determines whether the pump 1 is stopped and the temperature detected by the third temperature sensor 9 c is lower than the second threshold temperature. If the pump 1 is stopped and the temperature detected by the third temperature sensor 9 c is lower than the second threshold temperature, the processing proceeds to step S2. If the pump 1 is driven and/or if the temperature detected by the third temperature sensor 9 c is higher than or equal to the second threshold temperature, step S1 is performed again. The second threshold temperature can be a temperature higher than or equal to the boiling point of the working fluid at atmospheric pressure and, more specifically, a temperature higher than the boiling point. The second threshold temperature is, for example, a value obtained by adding a margin to the boiling point of the working fluid at atmospheric pressure. The margin is, for example, 0° C. or more and 5° C. or less.

In step S1 according to one modification, the controller 19 determines whether the pump 1 is stopped and the pressure detected by the first pressure sensor 8 a is lower than the third threshold pressure. If the pump 1 is stopped and the pressure detected by the first pressure sensor 8 a is lower than the third threshold pressure, the processing proceeds to step S2. If the pump 1 is driven and/or if the pressure detected by the first pressure sensor 8 a is higher than or equal to the third threshold pressure, step S1 is performed again. The third threshold pressure can be a pressure higher than or equal to the atmospheric pressure and, more specifically, a pressure higher than the atmospheric pressure. The third threshold pressure can be a pressure lower than or equal to the detected pressure of the first pressure sensor 8 a during the power generation operation and, more specifically, a pressure lower than the detected pressure. A value the same as the first threshold pressure can be adopted as the third threshold pressure.

In step S1 according to one modification, the controller 19 determines whether the pump 1 is stopped and the temperature detected by the first temperature sensor 9 a is lower than the third threshold temperature. If the pump 1 is stopped and the temperature detected by the first temperature sensor 9 a is lower than the third threshold temperature, the processing proceeds to step S2. If the pump 1 is driven and/or if the temperature detected by the first temperature sensor 9 a is higher than or equal to the third threshold temperature, step S1 is performed again. The third threshold temperature can be a temperature higher than or equal to the boiling point of the working fluid at atmospheric pressure and, more specifically, a temperature higher than the boiling point. The third threshold temperature can be a temperature lower than or equal to the detected temperature of the first temperature sensor 9 a during the power generation operation and, more specifically, a temperature lower than the detected temperature. The third threshold temperature is, for example, a value obtained by adding a margin to the boiling point of the working fluid at atmospheric pressure. The margin is, for example, a value within the range of 0° C. to 5° C.

In the above-described example, in step S4 according to the first to fifth embodiments, it is determined whether the pressure detected by the second pressure sensor 8 b is higher than or equal to the second threshold pressure Pth2. However, instead of making the determination, it may be determined whether the temperature detected by the second temperature sensor 9 b is higher than or equal to a fourth threshold temperature. Instead of making the determination, it may be determined whether the temperature detected by the first temperature sensor 9 a is higher than or equal to a fifth threshold temperature.

More specifically, in step S4 according to one modification, the controller 19 determines whether the temperature detected by the second temperature sensor 9 b is higher than or equal to the fourth threshold temperature. If the temperature detected by the second temperature sensor 9 b is higher than or equal to the fourth threshold temperature, the processing proceeds to step S5 or step S11. If the temperature detected by the second temperature sensor 9 b is lower than the fourth threshold temperature, step 34 is performed again, or the processing proceeds to step S6. The fourth threshold temperature can be a temperature higher than or equal to the boiling point of the working fluid at atmospheric pressure and, more specifically, a temperature higher than the boiling point. The fourth threshold temperature may be a temperature lower than or equal to the detected temperature of the second temperature sensor 9 b during the power generation operation and, more specifically, a temperature lower than the detected temperature. The fourth threshold temperature can be higher than the first threshold temperature. The fourth threshold temperature is, for example, a value obtained by adding a margin to the boiling point of the working fluid at atmospheric pressure. The margin is, for example, a value within the range of 0° C. and 5° C.

In step S4 according to one modification, the controller 19 determines whether the temperature detected by the first temperature sensor 9 a is higher than or equal to the fifth threshold temperature. If the temperature detected by the first temperature sensor 9 a is higher than or equal to the fifth threshold temperature, the processing proceeds to step S5 or step S11. If the detected temperature of the first temperature sensor 9 a is lower than the fifth threshold temperature, step 34 is performed again, or the processing proceeds to step 36. The fifth threshold temperature can be a temperature higher than or equal to the boiling point of the working fluid at atmospheric pressure and, more specifically, a temperature higher than the boiling point. The fifth threshold temperature can be a temperature lower than or equal to the detected temperature of the first temperature sensor 9 a during the power generation operation and, more specifically, a temperature lower than the detected temperature. The fifth threshold temperature can be higher than the second threshold temperature. The fifth threshold temperature is, for example, a value obtained by adding a margin to the boiling point of the working fluid at atmospheric pressure. The margin is, for example, a value within the range of 0° C. and 5° C.

In the above-described examples, in step S6, the controller 19 determines whether the pump operation time is greater than or equal to the threshold time Tth. The determination based on the pump operation time can be applied to step S4 according to the first, fourth, and fifth embodiments.

More specifically, in step 34 according to one modification of the first embodiment, the controller 19 determines whether the pump operation time is greater than or equal to threshold time Tth. If the pump operating time is greater than or equal to the threshold time Tth, the processing proceeds to step S5. If the pump operation time is less than the threshold time Tth, step S4 is performed again.

In step S4 according to one modification of the fourth and fifth embodiments, the controller 19 determines whether the pump operation time is greater than or equal to threshold time Tth. If the pump operating time is greater than or equal to the threshold time Tth, the processing proceeds to step S11. If the pump operation time is less than the threshold time Tth, step S4 is performed again.

In the above-described example, in step S8 according to the second and third embodiments, it is determined whether the pressure detected by the second pressure sensor 8 b is higher than or equal to the second threshold pressure Pth2. However, instead of making the determination, it may be determined whether the temperature detected by the second temperature sensor 9 b is higher than or equal to the fourth threshold temperature. Instead of making the determination, it may be determined whether the temperature detected by the first temperature sensor 9 a is higher than or equal to the fifth threshold temperature.

More specifically, in step S8 according to the one modification, the controller 19 determines whether the temperature detected by the second temperature sensor 9 b is higher than or equal to the fourth threshold temperature. If the temperature detected by the second temperature sensor 9 b is higher than or equal to the fourth threshold temperature, the processing proceeds to step 39. If the temperature detected by the second temperature sensor 9 b is lower than the fourth threshold temperature, step S8 is performed again.

In step S8 according to one modification, the controller 19 determines whether the temperature detected by the first temperature sensor 9 a is higher than or equal to the fifth threshold temperature. If the temperature detected by the first temperature sensor 9 a is higher than or equal to the fifth threshold temperature, the processing proceeds to step S9. If the temperature detected by the first temperature sensor 9 a is lower than the fifth threshold temperature, step S8 is performed again.

In the above-described examples, in step S1 according to the first to third embodiments, the controller 19 determines whether the pump 1 is stopped and the pressure detected by the second pressure sensor 8 b is lower than the first threshold pressure Pth1. However, in step S1, the controller 19 may determine whether the pump 1 is stopped, the pressure detected by the second pressure sensor 8 b is lower than the first threshold pressure Pth1, and the temperature detected by the fourth temperature sensor 9 d is higher than the detected temperature of the third temperature sensor 9 c. By determining whether the temperature detected by the fourth temperature sensor 9 d is higher than the temperature detected by the third temperature sensor 9 c, it can be determined whether the working fluid can be heated by the evaporator 2 when step S3 is performed. A temperature sensor may be provided for detecting the temperature of the evaporator 2 (more specifically, the temperature of a structural portion forming the evaporator 2), and the temperature detected by the temperature sensor may be used instead of the temperature detected by the fourth temperature sensor 9 d. Note that the same alteration can be applied to the fourth and fifth embodiments.

More specifically, in step S1 according to the one modification, in addition to determining whether the condition in step S1 according to the first to third embodiments is satisfied, the controller 19 determines whether the temperature detected by the fourth temperature sensor 9 d is higher than the temperature detected by the third temperature sensor 9 c. If the condition in step S1 according to the first to third embodiments is satisfied and the temperature detected by the fourth temperature sensor 9 d is higher than the temperature detected by the third temperature sensor 9 c, the processing proceeds to step S2. If the condition in step S1 according to the first to third embodiments is not satisfied and/or if the temperature detected by the fourth temperature sensor 9 d is lower than or equal to the temperature detected by the third temperature sensor 9 c, step S1 is performed again. Note that the same alteration can be applied to the fourth and fifth embodiments. Furthermore, the same alteration can be applied to the above-described modification.

In the above-described example, the bypass valve 5 and the shortcut valve 11 are variable flow rate valves. However, the bypass valve 5 and the shortcut valve 11 may be on-off valves, such as solenoid valves. The term “on-off valve” refers to a valve having an opening degree set to either 0% or 100%.

An electric ball valve can be adopted as the bypass valve 5 and/or the shortcut valve 11. The electric ball valve has a small change in the flow passage cross-sectional area between the valve portion and the pipe located before and after the valve. For this reason, according to the electric ball valve, the flow passage resistance when the working fluid circulates can be reduced.

In the power generation operation performed by the Rankine cycle apparatus, the working fluid circulates through the expander 3, thus enabling power generation by the expander 3 and the electric generator 18.

In contrast, it is not essential that the working fluid circulates through the expander 3 when the Rankine cycle apparatus is not generating power. In one specific example, after step S3 in FIGS. 3 and 5 and step S10 in FIG. 7, the working fluid circulates through the bypass circuit 16 in addition to the pump 1 and the evaporator 2. This configuration enables the working fluid to flow smoothly. After steps S3 and S10, the opening degree of the bypass valve 5 may be 100% or be less than 100%.

The opening degree 100% of the bypass valve 5 after steps S3 and S10 is advantageous from the viewpoint of smoothing the flow of the working fluid through the pump 1, the evaporator 2 and the bypass circuit 16. In this case, the working fluid is easily heated by the evaporator 2.

In contrast, the opening degree less than 100% of the bypass valve 5 after steps S3 and S10 is advantageous from the viewpoint of decreasing the temperature of the working fluid flowing into the condenser 4, as compared with an opening degree of 100%. In this case, the working fluid is easily condensed in the condenser 4, the gas-liquid two-phase working fluid is less likely to flow into the pump 1, and the pump 1 is less likely to experience a failure.

After step S3 and step S10, the inflow of the working fluid into the expander 3 can also be controlled by controlling the rotational speed of the expander 3. In the above-described example, a rotary shaft of the expander 3 and a rotary shaft of the electric generator 18 are connected to each other, Therefore, control of the rotational speed of the expander 3 can be achieved by controlling the rotational speed of the electric generator 18. The control of the rotational speed of the electric generator 18 can be achieved by the controller 19, for example. In one specific example, a PWM (Pulse Width Modulation) inverter (not illustrated) is connected to the electric generator 18. Then, the controller 19 PWM-controls the rotational speed of the electric generator 18 by using the PWM inverter. Note that during the power generation operation, the amount of power generation can be controlled by rotating the electric generator 18 under PWM control.

As described above, the bypass circuit 16 is connected to the portion of the circulation circuit 15 that is downstream of the evaporator 2 and upstream of the expander 3. A valve may be provided in a portion of the circulation circuit 15 that is located downstream of the above connection portion and upstream of the expander 3. In this way, inflow of the working fluid into the expander 3 can be prevented by closing the valve. For example, the valve can be closed in step S3 and step S10.

The fan 7 of the condenser 4 may be stopped during the first negative pressure prevention control, the second negative pressure prevention control, and the third negative pressure prevention control. This makes the working fluid less susceptible to the influence of the outside air temperature, and the pressure of the working fluid easily increases or does not easily decrease.

The fan 7 may be operated during the first negative pressure prevention control, the second negative pressure prevention control, and the third negative pressure prevention control. This makes it difficult for the gas-liquid two-phase working fluid to flow into the pump 1. For example, the fan 7 can be operated when the temperature of the working fluid flowing into the condenser 4 is high and the ability to condense the working fluid is to be improved.

In addition, the fan 7 can be controlled by using a sensor. In one example, the fan 7 is controlled on the basis of the detected value of the second pressure sensor 8 b and the detected value of the second temperature sensor 9 b. In one specific example, the degree of supercooling of the working fluid is calculated on the basis of the detected value of the second pressure sensor 8 b and the detected value of the second temperature sensor 9 b, and the fan 7 is controlled on the basis of the degree of supercooling. The term “control of the fan 7” is an expression that includes both control of the rotational speed of the fan 7 and control as to whether the fan 7 is driven or stopped.

Technique Related to Above Description

As can be understood from the above description, the Rankine cycle apparatus 21 to 23 according to the above description start the first control when the detected value of the sensor is lower than the first threshold value. The first control is control to cause the pump 1 to circulate the working fluid through the evaporator 2 and/or the heater 10. Such a Rankine cycle apparatus is suitable for preventing the pressure of the working fluid from becoming negative. This contributes to ensuring the reliability of the Rankine cycle apparatus. More specifically, the first control is control to cause the pump 1 to circulate the working fluid through the evaporator 2 and/or the heater 10 that is in a heat generation mode. In one specific example, the first control is control to cause the pump 1 to circulate the working fluid through the evaporator 2 in a working-fluid heatable mode and/or the heater 10 in a heat generation mode.

As used herein, the term “start of the first control” is a general idea that includes the start of driving of the pump 1 before the start of heat generation of the heater 10, the start of driving of the pump 1 that is simultaneous with the start of heat generation of the heater 10, and the start of driving of the pump 1 after the start of heat generation of the heater 10. In a typical example, the start of the first control and the start of driving the pump 1 are simultaneous.

In addition, the expression “circulating the working fluid by the pump 1 through the evaporator 2 and/or the heater 10” does not mean that the presence of the heater 10 is essential. As can be understood from the first embodiment, the first control can be performed without the heater 10.

In one specific example, the Rankine cycle apparatus 21 to 23 according to the above-described description start the first control to drive the pump 1 and circulate the working fluid through the pump 1 and the evaporator 2 when the pump 1 is stopped and the first condition is satisfied. The first control to circulate the working fluid through the evaporator 2 is started. Note that the first condition is that the detected value of the sensor is lower than the first threshold value.

The control method according to the above description includes a step of performing detection with a sensor. In addition, the control method includes a step of starting a first circulation in which the working fluid in a heated state is circulated by the pump 1 when the detected value of the sensor is lower than the first threshold value.

The expression “starting the first circulation in which the working fluid in a heated state is circulated by the pump 1” is described below. This expression refers to a general idea including the start of driving of the pump 1 before the heating of the working fluid, the start of driving the pump 1 that is simultaneous with the heating of the working fluid, and the start of driving of the pump 1 after the heating of the working fluid. In a typical example, the start of the first circulation and the start of driving the pump 1 are simultaneous.

More specifically, it can be said that the control method includes a step of heating the working fluid.

In the first circulation, the working fluid can be heated by the evaporator 2 and/or the heater 10, for example. The step of heating the working fluid can be performed by the evaporator 2 and/or the heater 10, for example.

The sensor may be a sensor that detects the pressure of the working fluid. In this case, the first condition is that the pressure of the working fluid is lower than the first threshold value. In this case, the first threshold value can be a pressure higher than or equal to the atmospheric pressure and, more specifically, a pressure higher than the atmospheric pressure. More specifically, in this case, the second pressure sensor 8 b can be used as the sensor, and the above-mentioned first threshold pressure Pth1 can be used as the first threshold value. Alternatively, in this case, the first pressure sensor 8 a can be used as the sensor, and the above-mentioned third threshold pressure can be used as the first threshold.

The sensor may be a sensor that detects the temperature of the working fluid. In this case, the first condition is that the temperature of the working fluid is lower than the first threshold value. In this case, the first threshold value may be a temperature higher than or equal to the boiling point of the working fluid at atmospheric pressure and, more specifically, a temperature higher than the boiling point. More specifically, in this case, the second temperature sensor 9 b can be used as the sensor, and the above-mentioned first threshold temperature can be used as the first threshold value. Alternatively, in this case, the first temperature sensor 9 a can be used as the sensor, and the above-described third threshold temperature can be used as the first threshold value.

The above-mentioned sensor may be a sensor that detects the temperature of the cooling medium to be heat-exchanged with the working fluid in the condenser 4. In this case, the first condition is that the temperature of the cooling medium is lower than the first threshold value. In this case, the first threshold value can be a temperature higher than or equal to the boiling point of the working fluid at atmospheric pressure and, more specifically, a temperature higher than the boiling point. More specifically, in this case, the third temperature sensor 9 c can be used as the sensor, and the above-mentioned second threshold temperature can be used as the first threshold value. Note that the term “temperature of the cooling medium to be heat-exchanged with the working fluid in the condenser 4” refers to the temperature of the cooling medium before being heat-exchanged with the working fluid in the condenser 4.

In one specific example, the sensor is configured to detect the pressure of the working fluid in the portion of the circulation circuit 15 that is downstream of the expander 3 and upstream of the pump 1 (that is, the second circuit 15 b). The pressure of the working fluid in this portion tends to be negative. For this reason, detection of the pressure of the working fluid in this portion by the sensor is suitable for preventing the pressure of the working fluid from becoming negative.

In a more specific example, the sensor is configured to detect the pressure of the working fluid in a portion of the circulation circuit 15 that is downstream of the condenser 4 and upstream of the pump 1 (that is, the third circuit 15 c).

In the first control, the working fluid may be circulated via the bypass circuit 16. In this way, the working fluid can be circulated, bypassing the expander 3 by using the bypass circuit 16. In this way, the working fluid can be circulated smoothly.

In one specific example, in the first control, the working fluid is circulated through the pump 1, the evaporator 2, and the bypass circuit 16. In this way, in the first control, the working fluid can smoothly circulate through the pump 1 and the evaporator 2.

Similarly, in the first circulation, the working fluid may pass through the bypass circuit 16. More specifically, in the first circulation, the working fluid may pass through the pump 1, the evaporator 2, and the bypass circuit 16.

Although not particularly limited, in the first control, the opening degree of the valve 5 of the bypass circuit 16 can be set to 50% or more and 100% or less. If the opening degree is set in this manner, the working fluid can be easily circulated smoothly through the pump 1 and the evaporator 2 in the first control. In the first control, the opening degree of the valve 5 of the bypass circuit 16 may be set to 75% or more and 100% or less.

Similarly, the first circulation may be performed with the opening degree of the valve 5 of the bypass circuit 16 set to 50% or more and 100% or less. The first circulation may be performed with the opening degree of the valve 5 of the bypass circuit 16 set to 75% or more and 100% or less.

In one example, in the first control, the working fluid is circulated through the evaporator 2 by the pump 1. If the detected value of the sensor is less than the second threshold value and the elapsed time from the start of the first control is the threshold time or more, the heater 10 starts generating heat. This configuration can mitigate the risk of the working fluid pressure becoming negative by using the heater 10, even when the first control cannot sufficiently mitigate the above-described risk. In a typical example, the second threshold value is greater than the first threshold value.

In one specific example, if the second condition is not satisfied and the pump operating time is greater than or equal to the threshold time, the second control is started in which the heater 10 is caused to generate heat and the working fluid is circulated via the pump 1 and the heater 10. Note that the second condition is that the above-mentioned detected value is greater than the second threshold value. The second threshold value is a threshold value greater than the first threshold. The pump operation time is the elapsed time from the start of driving the pump by the first control. This configuration can mitigate the risk of the working fluid pressure becoming negative by the second control using the heater 10, even if the first control cannot sufficiently mitigate the above described risk.

Similarly, in one example, the working fluid is heated by the evaporator 2 in the first circulation. The control method includes the step of starting heating the working fluid by the heater 10 if the detected value of the sensor is less than the second threshold value and the elapsed time from the start of the first circulation is the threshold time or more. In one specific example, the control method includes the step of causing the heater 10 to generate heat and starting the second circulation that circulates the working fluid through the pump 1 and the heater 10 if the second condition is not satisfied and the pump operation time is greater than or equal to the threshold time. In a typical example, the second threshold value is greater than the first threshold value.

The Rankine cycle apparatus may stop the driving of the pump 1 if the detected value is greater than or equal to the second threshold value. In this way, unnecessary power consumption of the pump can be avoided.

Similarly, the control method may further include the step of stopping the driving of the pump 1 if the detected value of the sensor is greater than or equal to the second threshold value.

As described above, the sensor may be a sensor that detects the pressure of the working fluid. In this case, the second condition is that the pressure of the working fluid is higher than or equal to the second threshold value. In this case, the second threshold value can be a pressure higher than or equal to the atmospheric pressure and, more specifically, a pressure higher than the atmospheric pressure. More specifically, the second pressure sensor 8 b can be used as the sensor, and the second threshold pressure Pth2 can be used as the above-described second threshold value. If the sensor is a sensor that detects the pressure of the working fluid, the first threshold value can be referred to as a first threshold pressure, and the second threshold value can be referred to as a second threshold pressure. In a typical example, the second threshold pressure is higher than the first threshold pressure.

As described above, the above-mentioned sensor may be a sensor that detects the temperature of the working fluid. In this case, the second condition is that the temperature of the working fluid is higher than or equal to the second threshold value. In this case, the second threshold value can be a temperature higher than or equal to the boiling point of the working fluid at atmospheric pressure and, more specifically, a temperature higher than the boiling point. More specifically, in this case, the second temperature sensor 9 b can be used as the sensor, and the above-mentioned fourth threshold temperature can be used as the second threshold value. Alternatively, in this case, the first temperature sensor 9 a can be used as the sensor, and the above-mentioned fifth threshold temperature can be used as the second threshold value.

In one specific example, in the second control, the working fluid is circulated through the pump 1, the heater 10, and the bypass circuit 16. Although not particularly limited, in the second control, the opening degree of the valve 5 of the bypass circuit 16 can be set to, for example, 50% or more and 100% or less. In the second control, the opening degree of the valve 5 of the bypass circuit 16 may be set to 75% or more and 100% or less.

Similarly, in one specific example, in the second circulation, the working fluid is circulated through the pump 1, the heater 10, and the bypass circuit 16. Although not particularly limited, the opening degree of the valve 5 of the bypass circuit 16 can be set to, for example, 50% or more and 100% or less in the second circulation. In the second circulation, the opening degree of the valve 5 of the bypass circuit 16 may be set to 75% or more and 100% or less.

The fluid circuit 14 may include the shortcut circuit 17 described above. In the second control, the working fluid may be circulated via the pump 1, the heater 10, and the shortcut circuit 17. Although not particularly limited, in the second control, the opening degree of the valve 11 of the shortcut circuit 17 can be set to, for example, 50% or more and 100% or less. In the second control, the opening degree of the valve 11 of the shortcut circuit 17 may be set to 75% or more and 100% or less.

Similarly, in the second circulation, the working fluid may be circulated via the pump 1, the heater 10, and the shortcut circuit 17. Although not particularly limited, the opening degree of the valve 11 of the shortcut circuit 17 can be set to, for example, 50% or more and 100% or less in the second circulation. In the second circulation, the opening degree of the valve 11 of the shortcut circuit 17 may be set to 75% or more and 100% or less.

In one specific example, the pump 1, the expander 3, and the condenser 4 are housed in a single enclosure. Since the pump 1, the expander 3, and the condenser 4 are housed in the enclosure, it is difficult for the pump 1, the expander 3, and the condenser 4 to exchange heat with the outside air, and the temperature is less likely to decrease due to the outside air. Therefore, the enclosure can exert an effect of preventing the working fluid from becoming negative.

The Rankine cycle apparatus according to the present disclosure can be applied to a direct contact Rankine cycle in which an evaporator is in direct contact with a heat source gas. In addition, the Rankine cycle apparatus according to the present disclosure can be applied to a binary Rankine cycle having a cycle of a water refrigerant or the like between a heat source gas and an evaporator. 

What is claimed is:
 1. A Rankine cycle apparatus comprising: a sensor; a pump; an evaporator; an expander; a condenser; and a fluid circuit through which a working fluid flows, the fluid circuit including a circulation circuit in which the pump, the evaporator, the expander, and the condenser are provided in this order, wherein the sensor is configured to detect one of (I) a pressure of the working fluid, (II) a temperature of the working fluid, and (III) a temperature of a cooling medium to be heat-exchanged with the working fluid in the condenser, wherein first control is started if a detected value of the sensor is less than a first threshold value, the first control is control to cause the pump to circulate the working fluid through the evaporator and/or a heater.
 2. The Rankine cycle apparatus according to claim 1, wherein (i) the sensor is configured to detect the pressure of the working fluid, and the first threshold value is a pressure higher than or equal to the atmospheric pressure, wherein (ii) the sensor is configured to detect the temperature of the working fluid, and the first threshold value is a temperature higher than or equal to the boiling point of the working fluid at atmospheric pressure, or wherein (iii) the sensor is configured to detect the temperature of a cooling medium to be heat-exchanged with the working fluid in the condenser, and the first threshold value is a temperature higher than or equal to the boiling point of the working fluid at atmospheric pressure.
 3. The Rankine cycle apparatus according to claim 1, wherein the sensor is configured to detect the pressure of the working fluid in a portion of the circulation circuit that is downstream of the expander and upstream of the pump.
 4. The Rankine cycle apparatus according to claim 1, wherein the fluid circuit includes a bypass circuit configured to connect a portion of the circulation circuit that is downstream of the evaporator and upstream of the expander to a portion of the circulation circuit that is downstream of the expander and upstream of the condenser, the working fluid is circulated through the bypass circuit in the first control.
 5. The Rankine cycle apparatus according to claim 4, wherein a valve is provided in the bypass circuit, an opening degree of the valve is set to 50% or more and 100% or less in the first control.
 6. The Rankine cycle apparatus according to claim 1, wherein the heater is provided in the fluid circuit; the working fluid is circulated through the evaporator in the first control, and the heater is configured to start generating heat if the detected value is less than a second threshold value and an elapsed time from the start of the first control is greater than or equal to a threshold time.
 7. The Rankine cycle apparatus according to claim 1, wherein driving of the pump is stopped if the detected value is greater than or equal to a second threshold value.
 8. The Rankine cycle apparatus according to claim 1, wherein the boiling point of the working fluid at atmospheric pressure is 0° C. or higher and 50° C. or lower.
 9. The Rankine cycle apparatus according to claim 1, further comprising: an electric generator configured to generate electric power by rotation torque of the expander.
 10. A method for controlling a Rankine cycle apparatus; the Rankine cycle apparatus configured to circulate a working fluid through a pump, an evaporator; an expander; and a condenser in this order, the method comprising: detecting, by using a sensor, one of (I) a pressure of the working fluid, (II) a temperature of the working fluid, and (III) a temperature of a cooling medium to be heat-exchanged with the working fluid in the condenser; and starting first circulation to cause the pump to circulate the working fluid in a heated state if a detected value of the sensor is less than a first threshold value.
 11. The method according to claim 10, wherein (i) the sensor is configured to detect the pressure of the working fluid, and the first threshold value is a pressure higher than or equal to the atmospheric pressure, wherein (ii) the sensor is configured to detect the temperature of the working fluid, and the first threshold value is a temperature higher than or equal to the boiling point of the working fluid at atmospheric pressure, or wherein (iii) the sensor is configured to detect the temperature of a cooling medium to be heat-exchanged with the working fluid in the condenser, and the first threshold value is a temperature higher than or equal to the boiling point of the working fluid at atmospheric pressure.
 12. The method according to claim 10, wherein the Rankine cycle apparatus includes a circulation circuit in which the pump, the evaporator, the expander, and the condenser are provided in this order, and wherein the sensor is configured to detect a pressure of the working fluid in a portion of the circulation circuit that is downstream of the expander and upstream of the pump.
 13. The method according to claim 10, wherein the Rankine cycle apparatus includes a circulation circuit in which the pump, the evaporator, the expander, and the condenser are provided in this order and a bypass circuit configured to connect a portion of the circulation circuit that is downstream of the evaporator and upstream of the expander to a portion of the circulation circuit that is downstream of the expander and upstream of the condenser, and wherein the working fluid passes through the bypass circuit in the first circulation.
 14. The method according to claim 13, wherein a valve is provided in the bypass circuit, an opening degree of the valve of the bypass circuit is set to 50% or more and 100% or less in the first circulation.
 15. The method according to claim 10, wherein the working fluid is heated by the evaporator and/or a heater in the first circulation.
 16. The method according to claim 10, wherein the working fluid is heated by the evaporator in the first circulation, and wherein the method further comprises starting heating the working fluid by the heater if the detected value is less than a second threshold value and an elapsed time from the start of the first circulation is greater than or equal to a threshold time.
 17. The method according to claim 10, further comprising: stopping the driving of the pump if the detected value is greater than or equal to a second threshold value.
 18. The method according to claim 10, wherein the boiling point of the working fluid at atmospheric pressure is 0° C. or higher and 50° C. or lower. 